This invention relates generally to tensed mask cathode ray tubes (CRTs) and particularly to a novel construction of such tubes.
Color CRTs, that is, those capable of displaying images in color, have been in existence for many years. The so-called shadow mask type of color CRT is most popular and is generally constructed with a target or screen consisting of a regular pattern of photo-deposited triads of red, blue and green light-emitting phosphors. The shadow mask is foraminous and is positioned a predetermined distance from the target, and by virtue of its pattern of apertures, effectively shadows all but selected ones of the individual light-emitting phosphors from its corresponding electron beam-emitting source located in the neck of the CRT. Since each CRT's shadow mask is used in photoscreening the phosphor target therein, means for enabling repeatable, precise interregistration of the shadow mask and the CRT faceplate is a necessity. In most CRTs, the shadow mask comprises a generally spherical foraminous metal sheet supported on a relatively heavy frame. The mask is capable of self-maintaining its configuration and is repeatably positioned in registration with the CRT panel by recourse to a plurality of studs embedded in the panel, and a corresponding plurality of stud-engaging flat springs attached to its supporting frame. The apparatus and screening techniques for such CRTs are all well-known in the art and will not be described further herein.
The limitations of the shadow mask with respect to power-handling capability and image resolution are well-known. This invention concerns a novel "tensed mask" CRT which utilizes a shadow mask in the form of a thin foil, with suitable apertures. The mask is held under tension in position adjacent to a flat or cylindrical phosphor-screen-bearing facepanel. Since the foil mask is extremely thin, etching of the apertures therein can be very precisely controlled, thus making a very high resolution tube an economic reality. Also, since the foil is under significant tension: that is, close to the elastic limit of the material used, it may be heated to a high temperature before the tension is relieved and mask distortion sets in. The electron energy that a tensed foil mask can absorb without significant degradation in color purity is much greater than that for a conventional spherical shadow mask. Consequently, a much brighter, sharper image is possible with a tensed mask CRT.
One of the inherent problems in a tensed mask CRT is that the support frame for the mask must be capable of withstanding the relatively large tension forces exerted by the mask thereon without substantial deformation. One mask support technique described and claimed in referent copending application Ser. No. 538,001, involves attaching the periphery of the mask to a glass support frame by means of a devitrifiable glass or "frit". The mask is typically composed of a steel that has a coefficient of thermal expansion selected to be significantly greater than that of glass. The mask is joined to the support frame under a sustained modest tension with a bead of devitrifiable frit. The assembly is heated to a temperature at which the frit devitrifies. During the heat cycle, the steel mask expands at a much greater rate than the glass support frame. When the frit devitrifies, it firmly bonds the periphery of the mask to the support frame. Upon cooling, the mask is prevented from returning to its original size and thereby is subjected to high tension forces--e.g., 30,000 psi.
As alluded to above, in accordance with conventional photographic techniques for fabricating the phosphor screens on the face panels of color CRTs, the shadow mask is used as a stencil and must therefore be repeatedly positioned in accurate registration with the panel and with respect to the light sources that stimulate the electron beams developed by the electron guns in the CRT. It will be appreciated that any deformation in the support frame for the tensed mask will result in corresponding distortions in the tensed mask aperture pattern during screening, and a corresponding distortion in the phosphor screen formed.
In accordance with referent copending applications, the tensed mask support frame ultimately becomes a structural element of the CRT envelope. During the assembly process, the entire CRT structure including panel, support frame and funnel, are subjected to an elevated temperature. When the frit devitrifies, a unified glass structure results that is hermetically sealed against the atmosphere. As the support frame and tensed mask are heated during the assembly process, the mask expands more than the frame, and the tension in the mask is greatly reduced. Deformation in the mask support frame is correspondingly reduced. As the devitrifiable frit vitrifies at the sealing temperature., the support frame is captured between the rigid panel and funnel portions of the CRT. Upon subsequent cooling, the tension forces are again exerted by the tensed mask, on the frame, but do not result in deformation of the support frame because it is now firmly supported by the face panel and funnel. Thus, the aperture pattern in the tensed mask of the finished CRT differs from the distorted aperture pattern in the mask when it was used to form the CRT phosphor screen. Because of this fact, color impurity is apt to occur.
In U.S. Pat. No. 3,166,211 to Stel et al, there is disclosed means for strengthening CRT funnel in which an implosion-resistant non-compressive coating extends over a portion of the funnel. The implosion-resistant coating is said to comprise a layer of strongly adherent synthetic resin containing wire-like fibers.
Logue in U.S. Pat. No. 3,720,345 discloses a television bulb with improved strength. The funnel side walls which extend generally parallel to the horizontal axis of the faceplate have a central portion thereof which is substantially flat in all cross sections, and is smoothly interconnected with the remaining portions of the side walls. The funnel side walls which extend generally parallel to the vertical axis of the faceplate have portions thereof which are substantially bowed outwardly relative to the vertical axis, and side walls which extend generally parallel to the vertical axis of the faceplate have portions thereof which are substantially bowed outwardly relative to the vertical axis, and side walls which extend generally parallel to the horizontal axis of the faceplate. The design is said to provide a television picture tube having substantially lower tensile stresses in the yoke area when the bulb is later evacuated.
U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, is directed to a method for processing a color cathode ray tube having a thin foil mask sealed directly to the bulb. Included in this disclosure is a description of the sealing of a foil mask between the juncture of the skirt of the faceplate and the funnel. The foil mask is noted as having a greater thermal coefficient of expansion than the glass to which it is mounted, hence following a heating and cooling cycle in which the mask is cemented at the funnel-faceplate juncture, the greater shrinkage of the mask upon cooling places it under tension. The mask is shown as having two or more alignment holes near the corners of the mask which mate with alignment nipples in the faceplate. The nipples pass through the alignment holes to fit into recesses in the funnel. In another embodiment, the front panel is shown as having an inner ledge forming a continuous path around the tube, the top surface of which is a Q-distance away from the faceplate for receiving the foil mask such that the mask is sealed within the tube envelope. An embodiment is also shown in which the faceplate is skirtless and essentially flat.
The following patents are also noted: 1,163,495 (GB); 2,761,990; 3,440,469; 3,638,063; 3,873,874; 3,894,321; 4,069,567; and 4,495,437.
In accordance with the present invention, the support frame comprises a major portion of the funnel of the CRT itself. Thus not only are the advantages of the above-mentioned copending applications achieved, but the funnel structure of the CRT may be significantly reduced in weight with an overall reduction of processing times and weight of the finished CRT.